1. Field of the Invention
The present invention relates to an enhanced surface laminar circuit board, and in particular, to a surface laminar circuit board in which a conductive layer of the surface laminar circuit board is removed in a region beneath a conductive pad.
2. Background Information
A printed circuit board (also known as a printed wiring board) is typically a flat board having a front surface that has various electronic components, such as integrated circuit chips, attached thereto and having electrically conductive pathways or wirings (also known as traces) between the components, which are printed on the back and/or front surface of the board.
The conventional printed circuit board is further typically formed of a plurality of superposed, laminated and alternating layers of conductive and insulating materials. The layers are each formed in discrete planes. For example, the conductive layers of a typical printed circuit board may include one or more internal wiring planes (i.e., a set of wirings located in one plane), each of which includes a number of individual conductive wirings. As mentioned above, the wirings are used to interconnect the various electronic components locatable on the printed circuit board together, and allow the transmission of electrical signals.
Further, the conductive layers of the printed circuit board may also include one or more power planes and/or ground planes, which are typically sheets of conductive material, such as copper. The power planes are used to supply power from the printed circuit board to the various electronic components located on the printed circuit board, whereas the ground planes serve as a ground potential for the various electronic components. The power planes and the ground planes may be located in different planes from the wiring planes, or may be located in the same plane as a respective wiring plane.
Each of the conductive layers of the printed circuit board are separated from the other overlying and/or underlying conductive layers by a respective layer of insulating material. Moreover, typically the printed circuit board will be provided with a number of plated mechanically-formed through holes and/or a number of plated mechanically-formed vias (i.e., blind holes formed in the insulating layers and plated or filled with a conductive material). Each plated via and plated through hole is electrically coupled to a respective conductive layer, and is used to transmit power or electrical signals, through respective insulating layers, to and from the associated electrical components and/or between the respective conductive layers.
In general, the electronic components can be attached to the surface of the printed circuit board in two different manners. If the electronic components are of the type known as a pin-through-hole part, the board will be provided with a plurality of holes therethrough. The pin-through-hole part has legs (pins) which fit through the holes, and are soldered, for example, to secure the component to the board. Alternatively, the parts may be attached to the board using surface mount technology (SMT). With this technology, the board, which may also include pin-through-hole-parts, is provided with a pad on a top or bottom surface thereof, on which a component lead is placed for securing the component (known as a surface mounted component) to the board. The procedure associated with SMT includes, in general, the use of a screen printer, and one or more component placement machines, for example, a high-speed placer, and a general purpose placement machine. The screen printer applies solder paste to a board, whereas the component placement machine populates the board with various components. After the board is completely populated with components, it is moved through an oven, where the solder paste is reflowed.
The aforementioned conventional printed circuit boards typically are limited in their density due to design constraints. Thus, so-called surface laminar circuit boards have been developed. Surface laminar circuit boards are tailored to accept surface mounted components, and have an increased density over a standard printed circuit board. Surface laminar circuit boards are particularly useful in the areas of data processing, consumer, and telecommunications, where miniaturization is a strong driver.
The surface laminar circuit board builds on the standard printed circuit board to provide a high-density surface layer. The density is achieved by substituting micro photo-vias for plated through holes and mechanically formed vias, with a two-to three-times reduction in land diameter.
The manufacturing process of the surface laminar circuit board begins with a standard printed circuit board, i.e., a circuit board that includes a plurality of conductive layers separated by an insulating layer, such as an industry standard FR4 insulating layer. Typically, the standard printed circuit board will have a conductive layer, which will serve at least as a signal ground layer (i.e., a ground plane), on its upper surface. A liquid or film photosensitive dielectric material is applied over the upper surface, that is, in contact with the underlying signal ground layer. The photosensitive dielectric material is then patterned through exposure and development of the dielectric material to form micro photo-vias to the conductive signal ground layer directly below. Through holes may then be drilled, followed by a plating and etch process or pattern plate process to form the circuitry (i.e., the pads and the signal traces of the surface laminar circuit board), with the dielectric layer separating the circuitry from the underlying signal ground layer.
This process can provide large numbers of small blind micro vias economically, consuming far less real estate than normal plated through holes and mechanically-formed vias. With blind micro photo-vias, real estate is consumed only on one side of the printed circuit board, whereas a plated through hole consumes real estate on all layers of the printed circuit board, even though the desired interconnection may be only from the top layer of the printed circuit board to the layer below. Further, the blind micro photo-vias of the surface laminar circuit board are smaller than mechanically-formed blind vias or plated through holes.
However, the known surface laminar circuit board has a high parasitic capacitance of the pads used to mount the surface mounted components. This is because the dielectric layer, which separates the signal ground layer from the pads and the signal traces (which are electrically coupled to the pads) is relatively thin. This places the signal ground layer very close to the signal traces (i.e., currently about 40 micrometers). This closeness causes the pads to have a higher capacitance to ground (i.e., parasitic capacitance) than with a standard printed circuit board, in which the signal traces and pads have a larger separation to the signal ground layer (i.e., the thickness of the insulating layer, which is about 100 micrometers or greater). This unwanted parasitic capacitance undesirably reduces frequency response. Therefore, there is need for a surface laminar circuit board mounting arrangement in which parasitic capacitance of the pad is reduced.
Moreover, the dielectric layer, which separates the signal ground layer from the signal traces and pads, does not bond as well to copper as it does to an insulating layer, such as the FR-4 insulating layer. However, the signal ground layer is typically a solid sheet of copper, and conventionally separates the underlying insulating layer from the overlying dielectric layer, so that the dielectric layer is formed on the signal ground layer. Typically, to increase the bonding strength of the dielectric layer to the copper signal ground layer, the signal ground layer is provided with a number of small holes. This allows a portion of the dielectric layer to bond directly to the insulating layer via the holes in the signal ground layer. However, this approach does not provide sufficient contact area between the dielectric layer and the signal ground layer to provide a suitable bond, especially in a region of the pads. Thus, the bond between the dielectric layer of the conventional surface laminar circuit board and the signal ground layer is relatively weak, resulting in a weak bond strength of the pad to the circuit board.
Therefore, there is need for a surface laminar circuit board mounting arrangement having an increased bond strength of the pad.
It is, therefore, a principle object of this invention to provide an enhanced surface laminar circuit board.
It is another object of the invention to provide an enhanced surface laminar circuit board that solves the above mentioned problems.
These and other objects of the present invention are accomplished by the enhanced surface laminar circuit board disclosed herein.
According to one aspect of the invention, and in order to reduce parasitic capacitance between a yet-to-be formed conductive pad and the underlying signal ground layer, and to improve the bonding strength of a yet-to-be deposited dielectric layer, a portion of the signal ground layer is removed, so as to form a hole in the signal ground layer. This hole is positioned so as to be in registration with the yet-to-be formed conductive pad, and is sized so that a major portion (over 50%, and preferably about 100%) of the conductive pad will be located in an area defined by the hole (i.e., either directly over or within the hole). Moreover, the hole exposes a substantial portion of the underlying insulating layer, which would conventionally be covered by the signal ground layer.
After the hole is formed in the signal ground layer, a liquid or film photosensitive dielectric layer is applied over the signal ground layer, so as to fill the hole.
In an exemplary aspect of the invention, the hole formed within the signal ground layer is larger than the surface area of the yet-to-be formed conductive pad. As such, the dielectric layer, which exhibits a poor bonding strength with copper, but good bonding capabilities to a conventional insulating material, will be in direct contact with, and positively bonded to the underlying insulating layer in a region of the hole. Thus, the dielectric layer is not as likely to delaminate as with the conventional arrangements. Therefore, this arrangement advantageously increases the bond of the dielectric layer to the underlying printed circuit board, especially in a region of the yet-to-be formed conductive pad.
In a further exemplary aspect the invention, the photosensitive dielectric layer is patterned to remove a portion of the dielectric layer, in a region of the hole, prior to forming the pads. As such, the pads can then be formed directly on the uppermost insulating layer. Since the conductive pads typically bond better to a conventional printed circuit board insulating material than to a photosensitive dielectric material, this arrangement advantageously further increases the bonding strength of the pads.
By removing the signal ground layer from directly under the pads, the cause of the high parasitic capacitance in the conventional surface laminar circuit board is eliminated. As such, a surface mounted component utilized with the present invention will have an increased frequency response.
Moreover, since the dielectric layer is bonded directly to the underlying insulating layer over a relatively large area, the bonding strength of the dielectric layer is increased, reducing the risk of delamination of the surface laminar circuit board, especially in a region of the pad.